kube-aws: fix shellcheck warnings in build script
kube-aws: fix shellcheck warnings in build script

https://github.com/koalaman/shellcheck

Quote variales to prevent word splitting/globbing.
Prefer $() to ``

Enable "strict mode": http://redsymbol.net/articles/unofficial-bash-strict-mode/

joshix/coreos-kubernetes,IDKA/coreos-kubernetes,amarruedo/coreos-kubernetes,amarruedo/coreos-kubernetes,danielfm/coreos-kubernetes,turbonomic/coreos-kubernetes,bnea-engineering/coreos-kubernetes,chen-anders/coreos-kubernetes,limres/coreos-kubernetes,bnea-engineering/coreos-kubernetes,limres/coreos-kubernetes,joshix/coreos-kubernetes,bnea-engineering/coreos-kubernetes,chen-anders/coreos-kubernetes,IDKA/coreos-kubernetes,chen-anders/coreos-kubernetes,ah45/coreos-kubernetes,IDKA/coreos-kubernetes,coreos/coreos-kubernetes,limres/coreos-kubernetes,amarruedo/coreos-kubernetes,coreos/coreos-kubernetes,turbonomic/coreos-kubernetes,danielfm/coreos-kubernetes,amarruedo/coreos-kubernetes,ah45/coreos-kubernetes,danielfm/coreos-kubernetes,ah45/coreos-kubernetes,turbonomic/coreos-kubernetes